In addition to the pattern of the photospheric horizontal fields, a relatively good match is also found for the vertical current densities.
Both the HMI observation and the MHD simulation display a dominance of the J\(_z\)  and B\(_z\)  of the same signs in each polarity of the bipole, with an elongated double-peaked J\(_z\)  pattern all along the PIL, as well as more extended patches at the ends of the sheared PIL. In the model, those extended patches correspond to the footpoints of the FR field lines (Figure REF  d). One difference, however, between the observation and the model is that with HMI, the extended patches in the negative polarity is more clearly visible than in the positive polarity (Figure REF  b). We argue that this difference is minor since it may be due to that fact that in the positive polarity, the swirling patterns of the vector fields are located in relatively weaker vertical fields than in the negative one (i.e. 500G in the former vs. 1500G in the later, Figure REF  a). With the same twist in both polarities, the weaker fields result in weaker current densities in the positive polarity. Another difference is that in the MHD simulation, some strong QSL-related current sheets surround the FR footpoint related extended patches ([1]}; [2]}). These are not visible with HMI and we argue that this is due to the limitations of the HMI data, from which current sheets can only be extracted during flares and with some processing of the data, as in [3]}; [4]}.
